Holddown plate for comminuting apparatus



May 28, 1968 J. s. ECKERT HOLDDOWN PLATE FOR COMMINUTING APPAHATUS Filed Nov. 4, 1965 FIG. 2

FIG.

INVENTOR. JOHN S. ECKERT flag& 469

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ATTORNEYS United States Patent O 3,385,529 HOLDDOWN PLATE FOR COMMINUTING APPARATUS John S. Eckert, Silver Lake, Ohio, assignor, by mesne assignments, to U.S. Stoneware Inc., a corporation of Massachusetts Filed Nov. 4, 1965, Ser. No. 506,341 1 Claim. (Cl. 241-46.15)

ABSTRACT OF THE DISCLOSURE I In comminuting apparatus having a rotatable agitator mounted in and spaced from the inner wall of a vessel, the space containing grinding balls and a slurry of material to be comminuted, a holddown plate operative to limit the expansion and displacement of the slurry and otherwise to counteract the adverse effects on eflicient grinding of centrifugal force.

This invention relates to an apparatus for comminuting particles and more specifically to a holddown plate for said comminuting apparatus. This application refers specifically to my pending application, Ser. No. 462,669, filed June 9, 1965, now abandoned, which is a Continuaton-in-part of my application, Ser. No., 303,736, filed Aug. 26, 1963, now abandoned.

In the above identified, ending application, a comminuting vessel (or several vessels if needed) is filled with a slurry comprising particles of material to be ground, Suspended in a suitable liquid, and grinding balls or elements. An agitator with one or several horizontal intensifying rings operates rotatively within the vessel, agitating the slurry and causing the particles to be ground between the grinding balls.

One of the major problems related to the use of these rotary motion to a ball with which it is in contact; this r ball in turn rotates an adjacent -ball, etc. It is therefore an essential principle of the Operating design of this comminuting apparatus that the grinding balls remain in continuous contact with each other so they may rotate, and in so rotating crush 'between them the particles to be ground.

Because of the continuous contact between them, the grinding balls generate shear forces in a range of tons per square inch. This permits the mill to produce an excellent grinding action, and properly grind the particles Suspended in the slurry or mixture.

In addition to establishing that the grinding efl'icacy of the comminuting apparatus decreases as material viscosity increases, it has further been recognized that a loss of grinding ability or grinding action is also caused by a slight separation between the grinding elements or balls in the expanded mixture or slurry. In other words, as grinding takes place and grinding speed increases, centrifugal force tends to move the grinding balls slightly away from the rotor. This creates two problems: (1) that the rotor can no longer impart rotary motion to the grinding balls, and (2) that this results in a slight "Ice expansion of the entire mixture or slurry and decreased grinding eifectiveness.

The reason for this is that at first, grinding improves as the speed increases-but only up to a certain point. This point is reached when the centrifugal action of the balls becomes large enough so the balls adjacent to the agitator no longer contact the agitator, since all the shear is Originally developed between the first layer of balls next to the agitator and the agtator itself. Thus, as centrifugal force reaches a certain level, the balls and slurry tend to freeze" in the outer part of the vessel and no longer cause shearing agitation.

This serious problem can be overcome by 'adding a member to the vessel such as, for instance, a holddown plate or holddown cover with which it is possible to partially confine or contain the slurry mixture and prevent the grinding elements therein from expanding to -a detrmental level. This containing action tends to reduce the viscosity of the material in the mili and makes -it possible to maintain a highly efiicient grinding operation and reduce the amount of power required to accomplish grinding.

When the agtator first begins to rotate, the grinding elements assume a generally tetrahedral stack type arrangernent. However, as speed and centrifugal force increase, the tetrahedral array tends to change to a cubic type configuration. This Shift may cause an expansion of the slurry of from five percent to ten percent.

The novelty of this invention is that the holddown plate provides enough expansion Volume for the slurry to prevent the agitator and slurry from becoming one solid mass and freezng, yet not enough to reduce materially the grinding eificiency of the comminuting -apparatus.

The purpose of the holddown plate or cover is thus to prevent uncontrolled expansion, and insure uniform contact between grinding balls at all times. The holddown cover does not `allow the contact pressure within the vessel to vary during the grinding operation and does result `in uniform, even comminuting.

With the foregoing problems in mind, it is an objec of this invention to provide a holddown plate for a comminuting apparatus that permits even, uniform grinding.

It is another object of this invention to provide a grinding apparatus holddown cover that retains `and maintains an even and uniform grinding process as the grinding speed in the vessel increases.

It is another object of this invention to provide a holddown plate for a comminutng apparatus which permits even and uniform grinding when the comminuting apparatus reaches its optimum grinding speed.

It is another object of this invention to provide a cover for a comminuting apparatus which permits carrying out the grinding operation with the maximum of grinding efficency.

Other objects and aspects of the invention will become apparent from the following description of the preferred form of the invention shown and described ir `the specification and drawings, which form a part of the specification, and Wherein similar parts are identified by like reference characters in the various parts of the drawings.

FIGURE 1 is -an elevational detail which shows the holddown plate in section mounted on top of a comminuting vessel.

FIGURE 2 is a plan View of the holddown plate.

In referring to the drawings, it is to be understood that the same equipment and process can be used for dry grinding and emulsifying, except that usually only a central vessel is used for the latter process. In this description, the process of wet grinding a solid in a liquid will be described more particularly.

Referring to FIGURE 1, there is shown a cylindrical vessel 30 having inner wall 31 and outer wall 32, and

rotatable drive shaft 17 extending into the vessel for engagement with axially disposed agitator 33 adapted with radial intensifying rings 34. Cavity 35 between inner Wall 31 and agitator 33 is adapted to contain grinding balls 36 and a slurry (not illustrated) of material to be comminuted.

In FIGURE 1, numeral 10 represents the holddown plate. The holddown plate comprises a top circular member 12 provided with aperture 12a connected to a bottom member 18 by means of intermediate Wall members 15. The height of the holddown cover is designed so that when it -is Secured to the comminutng vessel, an optimum clearance 13 is created between the underside of member 18 and the top surface of the slurry in the vessel A collar-like structure 14 is connected centrally to member 12. This collar-like structure is provided with passage 14a which registers with aperture 12a, and both receive drive shaft 17. The space between the shaft and collar 14 accommodates a packing material 16 designed to retain the slurry or mixture within the vessel and prevent it from leaking out from the latter along the vessePs drive shaft 17. A holding nut 19 adapted with an axial passageway therethrough is received within the top portion of collar 14 to retain packing 16.

It must he remembered that while extreme expansion of the slurry and the consequent change in viscosity is detrimental to maintaining proper, uniform and even grinding, it is, -however, essential that a controlled clearance be assured and provided for a limited, initial expansion. The purpose for this provision is to avoid excessive friction within the vessel and prevent the mixture from "locking" the agitator. If room for a small amount of expansion were not allowed, the agitator and slurry would freeze" into one solid mass, preventing a l rotary motion.

The holddown plate is adapted with a circumferential flange 20 which is fastened to the comminuting vessel by means of bolts 22. Gasketing means 24 between the vessel and the holddown plate prevents leakage or seepage of the fluid out of the comminuting apparatus.

I claim:

l. In a comminuting apparatus comprsing, in com- 'bination, a cylindrical vessel having an inner and outer wall, a rotatable shaft extending into said vessel, for engagement with an axially disposed agitator adapted with radial intensifying rngs, a cavity between the inner wall of said vessel and said agitator adapted to contain grinding balls and a slurry of material to be comminuted, the improvement comprising means operative to counteract the adverse effects on efficent grinding of centrifugal force, including a holddown plate having a top portion and a bottom portion interconnected by an intermediate wall, means scaling said top portion to the top of said vessel, said bottom portion being disposed within the nner wall of said vessel, a centrally disposed aperture in said top portion, a collar portion extendng upwardly from said top portion and having a passageway therethrough registering with said 'aperture, said passageway and said aperture receiving said agitator shaft, sealing means disposed between said collar portion and said shaft, and retaining means received within said collar to compress and retain said scaling means.

References Cited UNITED STATES PATENTS 2,855,156 10/1950 Hochberg et al. 241-46 X 3,149,789 9/1964 S`zeguari 241-27 3,172,609 3/1965 Olsen et al. 241-172 X WILLIAM W. DYER, JR., P''mary Exam'er,

W. D. BRAY, Assistant Exam'ner. 

